Generally, metallized film capacitors comprise two tightly wound sheets, wrapped around a core, with each sheet including a metallized layer on one side, with the metallization absent from two oppositely disposed margin layers. The opposite ends of the rolled capacitor are sprayed with a conductive metal which bonds with the sheet having a metallized end, with leads attached to form the capacitor electrodes. The rolled capacitor is then disposed in a housing and impregnated with a dielectric fluid. Metallized film capacitors differ in the degree of impregnation from paper capacitors because of the tightness of the winding and the lack of porosity of the film, with some impregnation occurring through the wound ends. While metallized film capacitors are preferred due to their less labor intensive assembly, they have limited usefulness at higher voltages due to metal degradation.
Aliphatic or aromatic esters have been disclosed for use as dielectric capacitor impregnants, with the aromatic esters preferred in many applications. However, the aromatic esters have the disadvantage of exhibiting a high power factor at elevated temperatures, and suffer from degradation through hydrolysis. Various attempts have been made to overoome these disadvantages. In U.S. Pat. No. 3,754,173 to Eustanoe, an epoxide stabilized liquid aromatic ester is disclosed for use as an electrical capacitor impregnant in a paper or soggy foil capacitor, with the preferred ester being di(2-ethylhexyl) phthalate or DOP. The epoxide stabilizer increases the high temperature life of the capacitor by interacting with those chemical compounds ordinarily found in or generated during operation of the electrical capacitors, preventing these compounds from degrading the DOP. Eustance states that those epoxides of high molecular weight are preferred in greater amounts than those epoxides of lower molecular weight, with generally amounts between about 0.1% by weight up to about 10% being satisfactory. In the three examples, Eustance uses 1.0% by weight of diglycidyl ether of bisphenol A to stabilize the DOP in a paper capacitor.
While Eustance teaches 0.1-10% generally for paper and soggy foil capacitors, common practice in the industry is to use between 0.5 and 1% by weight of an epoxide in paper capacitors with complete impregnation. While useful in impregnatable capacitors, these aromatic ester dielectrics, even with epoxide stabilizers, have not proved useful in metallized film capacitors, which cannot be totally impregnated. For such capacitors, most efforts are shifting to other dielectric compositions including propylene glycol (U.S. Pat. No. 4,422,962 to Cichanowski), isocyanates (U.S. Pat. No. 4,317,159 to DeQuasie), or glyceryl triacetate with 30 to 90% volume epoxidized soybean oil (U.S. Pat. No. 4,642,731to Shedigian).
However, the search continues for high performance dielectric fluids for use with metallized film capacitors which allows their utilization at higher voltages than previously achieved, providing enhanced performance, stability, and life.